Reactive gate electrode conductive barrier

ABSTRACT

A method, and corresponding transistor structure are provided for protecting the gate electrode from an underlying gate insulator. The method comprises: forming a gate insulator overlying a channel region; forming a first metal barrier overlying the gate insulator, having a thickness of less than 5 nanometers (nm); forming a second metal gate electrode overlying the first metal barrier, having a thickness of greater than 10 nm; and, establishing a gate electrode work function exclusively responsive to the second metal. The second metal gate electrode can be one of the following materials: elementary metals such as p+ poly, n+ poly. Ta, W, Re, RuO2, Pt, Ti, Hf, Zr, Cu, V, Ir, Ni, Mn, Co, NbO, Pd, Mo, TaSiN, and Nb, and binary metals such as WN, TaN, and TiN. The first metal barrier can be a binary metal, such as TaN, TiN, or WN.

RELATED APPLICATIONS

This application is a continuation-in-part of an application entitled,MOSFET THRESHOLD VOLTAGE TUNING WITH METAL GATE STACK CONTROL, inventedby Gao et al., filed Jan. 15, 2003, Ser. No. 10/345,744 now U.S. Pat.No. 6,861,712.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to CMOS fabrication processes and, moreparticularly, to a CMOS device that uses a stacked metal structure, witha metal barrier to protect a metal gate electrode, that does not affectthe gate electrode work function.

2. Description of the Related Art

The threshold voltages (V_(th)) of the NMOS and PMOS components in acomplementary metal oxide semiconductor (CMOS) circuit largely dictatethe speed, standby current, and operating current performancecharacteristics. The V_(th) must be set to maximize the “on” current,while minimizing the “off” current. Usually this is a trade off that isdetermined by the circuit design and application. Typically, the V_(th)is adjusted through fine-tuning of the doping level in the channelregion of the transistors with a V_(th) adjust implant. As the featuresize of transistors continues to scale down, the struggle to minimizeshort channel effects, and reduce punchthrough and drain-induced barrierlowering with implantations and anneals, ultimately limit the devicespeed.

As an alternative method of adjusting V_(th), the work function of thegate can be controlled. This is usually done with implants into the gatepolysilicon, where donor type dopant is placed in the gate for NMOS, andacceptor dopants into PMOS gates. The use of doped polysilicon gatespresents a different set of problems, however. Dopant diffusion, throughthe gate dielectric into the channel, affects the V_(th) polysilicondepletion near the gate dielectric, and limits the performance of thetransistors.

Doped polysilicon has been the gate material of choice for the lastseveral generations of microelectronics technology. To achieve lowV_(th) devices (required for high performance), p+ poly is used for PMOSand n+ poly is used for NMOS. As devices are scaled, the thickness ofthe poly-Si gate is decreased. In order to maintain low sheet resistanceand a large effective oxide capacitance (i.e. minimize poly depletioneffects) it has been necessary to increase the poly doping density witheach successive generation. This has led to the problem of channelautodoping in which boron (B) from the gate poly diffuses across thethin gate dielectric and into the channel, causing V_(th) variationsthat degrade device performance.

This diffusion, or autodoping problem is addressed with the use of metalgate materials. With metal gate technologies, the choice of anappropriate work function material is necessary for the N and P MOSFETs.The work function is the energy required to remove an electron from theFermi level to vacuum. The work function of different materials, anddifferent metals, varies. Since the NMOS and PMOS work functionrequirements are different, the metal materials are typically different.Thus, dual metals, with work functions corresponding to p+ and n+ polySi) will be required for CMOS circuits.

However, the use of completely different metal materials for NMOS andPMOS gates results in additional fabrication steps and undesiredcomplexity.

It has been shown that many of the desirable gate metal materials haveadhesion and/or stability problems when placed in direct contact withthe SiO₂ or high k gate dielectrics, such as HfO₂ or ZrO₂. For example,it is known that Pt does not adhere well to SiO₂, and metals such as Ti,Hf, or Zr, scavenge O, reducing the underlying dielectric film, causingdegradation and increased leakage.

It would be advantageous if the above-mention diffusion and adhesionproblems could be addressed using a gate electrode diffusion barrier.

It would be advantageous if a gate electrode diffusion barrier could beused that didn't contribute to the work function of a metal gateelectrode.

It would be advantageous if a gate electrode diffusion barrier wereconductive, so that it did not contribute to the capacitance of the gatestack.

SUMMARY OF THE INVENTION

As noted above, metal gates are needed in MOS fabrication processes.Conventionally, metals are placed in direct contact with the gatedielectric, such as SiO2 or high-k dielectrics. The direct contact ofthe gate electrode metal and gate dielectric can result in adhesion andstability issues, as the reactivity of metal with the dielectric maylead to oxygen scavenging.

The present invention discloses the use of a thin interfacial layer orbarrier that improves adhesion between a metal gate and an underlyingdielectric. The layer also serves as a barrier metal to reducereactivity. As long as the thickness of this interfacial metal is small,it does not impact Vth of the transistor. Because the barrier/layer is ametal, gate resistivity and capacitance are not adversely impacted.

Accordingly, a method is provided for protecting the gate electrode froman underlying gate insulator. The method comprises: forming a gateinsulator overlying a channel region; forming a first metal barrieroverlying the gate insulator, having a thickness of less than 5nanometers (nm); forming a second metal gate electrode overlying thefirst metal barrier, having a thickness of greater than 10 nm; and,establishing a gate electrode work function exclusively responsive tothe second metal.

The second metal gate electrode can be one of the following materials:elementary metals such as p+ poly, n+ poly, Ta, W, Re, RuO2, Pt, Ti, Hf,Zr, Cu, V, Ir, Ni, Mn, Co, NbO, Pd, Mo, TaSiN, or Nb, and binary metalssuch as WN, TaN, or TiN. The second metal gate electrode can have eithera high or low work function, depending upon the choice of gate electrodemetal. The gate insulator can one of the following materials: SiO2 andhigh-k dielectrics such as HfO2, ZrO2, Al2O3, La2O3, HfAlOx, or HfAlON,and binary, ternary, or nitrided metal oxides. The first metal barriercan be a binary metal, such as TaN, TiN, or WN.

The first barrier metal prevents the migration of oxygen from the gateinsulator to the second metal gate electrode. If the gate electrode is ap+ poly material, the first barrier metal can prevent the migration of Binto the gate insulator, from the p+ poly gate electrode.

Additional details of the above-described method and a MOSFET transistorwith a reactive metal gate electrode barrier, are explained below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional view of the present invention MOSFETtransistor with a reactive metal gate electrode barrier.

FIG. 2 is a partial cross-sectional view of a step in the fabrication ofthe present invention transistor of FIG. 1.

FIG. 3 is a flowchart illustrating the present invention method forprotecting the gate electrode from an underlying gate insulator, in aMOSFET transistor with a reactive metal gate electrode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a partial cross-sectional view of the present invention MOSFETtransistor with a reactive metal gate electrode barrier. The transistor100 comprises a channel region 102. Also shown are a source 104 anddrain 106. A gate insulator 108 overlies the channel region 102. A firstmetal barrier 110 overlies the gate insulator 108. A second metal gateelectrode 112 overlies the first metal barrier 110. The second metalgate electrode 112 has a gate electrode work function exclusivelyresponsive to the second metal 112. That is, the first metal barrier110, despite being a metal, or metal-like material, does not contributeto, or impact the gate work function.

Beneficially, the first barrier metal 110 prevents the migration ofoxygen from the gate insulator 108 to the second metal gate electrode112. The first barrier metal 110 also prevents the migration ofmaterials from the gate electrode 112, into the gate insulator 108. Forexample, if a p+ poly gate electrode 112 is used, the first barriermetal 110 prevents the migration of B into the gate insulator 108 from ap+ poly gate electrode 112. Likewise, the first metal barrier 110prevents the migration of dopants if an n+ poly gate electrode isformed.

The first metal barrier 110 has a thickness 114 of less than about 5nanometers (nm) and the second metal gate electrode has a thickness 116of greater than about 10 nm. In some aspects, the first metal barrier110 has a thickness 114 of greater than 1.5 nm, and less than 5 nm. Itshould be noted that as the second metal thickness 116 increases, theeffect of the first metal thickness 114 becomes less significant in thedetermination of work function. As is understood by those skilled in theart, the transistor 100 has a threshold voltage (V_(th)) responsive tothe second metal gate electrode 112 work function. The threshold voltagemay also be responsive to the thickness of dielectric 108 and the dopingof underlying silicon substrate 120.

The second metal gate electrode 112 can be formed from elementary metalssuch as W, Ta, Re, RuO2, p+ poly, n+ poly, Pt, Ti, Hf, Zr, Cu, V, Ir,Ni, Mn, Co, NbO, Pd, Mo, TaSiN, or Nb. Although p+ and n+ poly are notactually elementary metals, they have been included in this categorybecause they have characteristics of an elementary metal. Further,binary metals such as WN, TaN, or TiN can be used as the second metalgate electrode 112.

The gate insulator 108 can be either SiO2 or a high-k dielectrics suchas HfO2, ZrO2, Al2O3, La2O3, HfAlOx, or HfAlON. Further, binary,ternary, or nitrided metal oxides can be used as the gate insulator 108.The first metal barrier 110 can be a binary metal, such as TaN, TiN, orWN. It should be noted that these first metal materials can also be usedas a second metal gate electrode in some aspects. Depending on thebinary metal stoichiometry, in this case the amount of N added, thebinary metal work function can be made to vary. When used with secondmetal materials, the thickness of the first metal material, regardlessof the stoichiometry, prevents a contribution to the gate electrode 112work function.

More specifically, if the second metal gate electrode 112 has a highwork function, then following materials may be used: elemental metalssuch as Ir, Pt, Cu, Re, Co, Ni, Mn, RuO2, p+ poly, Pd, Mo, or TaSiN.Further, binary metals such as TaN, WN, or TiN may be used. A high workfunction is about 5eV, +/−a few tenths of an eV.

If the second metal gate electrode 112 has a low work function, then thefollowing materials may be used: elementary metals such as Al, Nb, Hf,Zr, V, Ir, n+ poly, W, Ti, NbO, or Ta. Further, binary metals such asTaN, TiN, or WN may be used. Low work function metals are generallyreactive, and the use of the first metal barrier with reactive gateelectrode materials is especially beneficial. A low work function isgenerally about 4eV, +/−0.2 eV.

The first metal barrier 110 and the second metal gate electrode arenon-diffused metals. As used herein, the term “non-diffused” is intendedto mean that the two metal layers are not intentionally diffused by anannealing process, for example, an annealing process that ensures thatthe two metals are fully mixed—reaching an equilibrium state. Generallyspeaking, metals do diffuse when they contact. However in somecircumstance, like with a compound/metal such as TiN/Pt is used, aninsignificant amount of diffusion may occur. Overlying metals with aninsignificant amount of diffusion are still considered to benon-diffused. Alternately stated, when metal combinations such as Ti/Ptare used, where a small degree of diffusion is inevitable, this partialdiffusion does not contradict the more general non-diffused state of themetals.

Functional Description

As noted above, the present invention uses a thin interfacial (firstmetal) layer, or barrier to improve adhesion between a metal gate and anunderlying dielectric, and to serve as a barrier metal to reducereactivity. A key element is that a thin interfacial metal does notimpact V_(th) of the transistor. Because the barrier/layer is a metal,gate resistivity and capacitance are not adversely impacted.

FIG. 2 is a partial cross-sectional view of a step in the fabrication ofthe present invention transistor of FIG. 1. The structure, prior todepositing the first metal barrier layer, is formed by any state of theart method. FIG. 2 is an exemplary gate replacement process. However,the present invention is not limited to any particular gate formationprocess. A gate dielectric, of any kind, has been deposited or grown,and is ready for the metal gate.

A thin first metal barrier/interfacial layer of metal is deposited byany state of the art method. Depending on the structure, this metalmaterial can be TiN, TaN or WN.

A (second) metal gate with the desired work function properties is thendeposited to desired thickness by any state of the art method. Dependingof the design of the structure, this film can be Pt, Ti, Hf, Zr, or anyother potential metal or metal compound. The metal stack is then etchedor chemical mechanically polished (CMP'd) to form the desired devicestructure. The structure can then be treated thermally, electrically, ormechanically, as required.

The first metal barrier improves adhesion and reduces any reactionbetween the overlying gate and the underlying dielectric. Because theinterfacial layer is thin, typically less than 50 Å the work function ofthe transistor device is determined solely by the top (second) gatemetal.

A gate stack consisting of any gate metal and any dielectric can be madecompatible, providing a suitable barrier metal is used. In some aspects,if the work function of the metal gate is not an optimum value, thethickness of the first metal barrier can be adjusted, typically to avalue greater than 90 Å, to shift the threshold voltage to the desiredvalue.

FIG. 3 is a flowchart illustrating the present invention method forprotecting the gate electrode from an underlying gate insulator, in aMOSFET transistor with a reactive metal gate electrode. Although themethod is depicted as a sequence of numbered steps for clarity, no ordershould be inferred from the numbering unless explicitly stated. Itshould be understood that some of these steps may be skipped, performedin parallel, or performed without the requirement of maintaining astrict order of sequence. The method starts at Step 300.

Step 302 forms a gate insulator overlying a channel region. Step 304forms a first metal barrier overlying the gate insulator. Step 306 formsa second metal gate electrode overlying the first metal barrier. Step308 establishes a gate electrode work function exclusively responsive tothe second metal. In some aspects, Step 308 establishes a thresholdvoltage (V_(th)).

In some aspects of the method, forming a first metal barrier (Step 304)includes forming a first metal barrier having a thickness of less thanabout 5 nanometers (nm). Forming a second metal gate electrode (Step306) includes forming a second metal gate electrode having a thicknessof greater than about 10 nm. In other aspects, Step 304 forms a firstmetal barrier having a thickness of greater than 1.5 nm, and less than 5nm.

Generally, forming a second metal gate electrode (Step 306) includesforming a second metal gate electrode from a material such as elementarymetals including p+ poly, n+ poly, Ta, W, Re, RuO2, Pt, Ti, Hf, Zr, Cu,V, Ir, Ni, Mn, Co, NbO, Pd, Mo, TaSiN, or Nb, and binary metalsincluding WN, TaN, or TiN. This is a list of commonly used materials,and it is not intended to be an exhaustive list of every possiblematerials. Other materials would be known by those skilled in the art.

Forming a gate insulator overlying a channel region (Step 302) includesforming a gate insulator from a material such as SiO2, high-kdielectrics such as HfO2, ZrO2, Al2O3, La2O3, HfAlOx, or HfAlON, andbinary, ternary, or nitrided metal oxides. Forming a first metal barrier(Step 304) includes forming the first metal barrier from a binary metalsuch as TaN, TiN, or WN. Again, these are not intended to be exhaustivelists of every possible material that can be used.

If Step 306 forms a second metal gate electrode having a high workfunction, then the second metal may be elemental metals such as Ir, Pt,Cu, Re, Ni, Mn, Co, RuO2, p+ poly, Pd, Mo, or TaSiN, or a binary metalsuch as TaN, WN, or TiN. If Step 306 forms a second metal gate electrodehaving a low work function, then the second metal may be an elementarymetals such as Al, Nb, Hf, Zr, V, Ir, n+ poly, W, Ti, Ta, or NbO, or abinary metal such as TaN, TiN, or WN.

In some aspects, forming a first barrier metal overlying the gateinsulator in Step 304 includes the first metal barrier preventing themigration of oxygen from the gate insulator to the second metal gateelectrode. In other aspects, Step 304 prevents the migration of B intothe gate insulator from a p+ poly gate electrode.

Stacked metal gate MOSFET devices and associated fabrication processeshave been presented above. For full CMOS applications, where the metalstack for the NMOSFET is different from the PMOSFET, the first metallayer is typically deposited over the entire wafer surface. Then,patterning and etching steps are performed. Since the first metalbarrier is extremely thin, less than 5 nm, it can be etched easily byeither a wet or dry process.

Examples have been given of various gate metals and first and secondgate metal combinations. However, the invention is not limited to simplythese examples. Further, examples have been given using only metal gatematerials. The invention can also be enabled using other materials, orcombinations of metals with other materials. For example, the firstlayer may be a metal and the second layer polysilicon. Other variationsand embodiments of the invention will occur to those skilled in the art.

1. In a MOSFET transistor with a reactive metal gate electrode, a methodfor protecting the gate electrode from an underlying gate insulator, themethod comprising: forming a gate insulator overlying a channel region;forming a gate electrode including: a first metal layer, which is abarrier, overlying the gate insulator, having a thickness of less than 5nanometers (nm); and, a second metal layer overlying the first metallayer; wherein the gate electrode has a work function exclusivelyresponsive to a second metal layer, selected from a group consisting ofPt, NbO, Pd, and Nb.
 2. The method of claim 1 wherein forming a gateelectrode including a second metal layer includes forming a second metallayer having a thickness of greater than about 10 nm.
 3. The method ofclaim 2 wherein forming a gate electrode including a first metal layerincludes forming a first metal layer having a thickness of greater than1.5 nm, and less than 5 nm.
 4. The method of claim 1 wherein forming agate insulator overlying a channel region includes forming a gateinsulator from a material selected from the group consisting of SiO2,high-k dielectrics such as HfO2, ZrO2, Al2O3, La2O3, HfAlOx, and HfAlON,and binary, ternary, and nitrided metal oxides.
 5. The method of claim 1wherein forming a gate electrode including a first metal layer includesforming the first metal layer from a material selected from the groupconsisting of binary metals such as TaN, TiN, and WN.
 6. The method ofclaim 5 wherein forming a gate electrode includes forming a gateelectrode having a high work function.
 7. The method of claim 6 whereinforming a gate electrode with a high work function includes forming agate electrode including a second metal layer being selected from thegroup consisting of Pt and Pd.
 8. The method of claim 5 wherein forminga gate electrode includes forming a gate electrode having a low workfunction.
 9. The method of claim 8 wherein forming a gate electrode witha low work function includes forming a gate electrode including a secondmetal layer selected from the group consisting of Nb and NbO.
 10. Themethod of claim 1 wherein forming the gate electrode work functionexclusively responsive to the second metal layer includes establishing athreshold voltage (Vth).
 11. The method of claim 1 wherein forming agate electrode including a first metal layer overlying the gateinsulator includes the first metal layer acting as a barrier to preventthe migration of oxygen from the gate insulator to the second metallayer.
 12. In a MOSFET transistor with a reactive metal gate electrode,a method for protecting the gate electrode from an underlying gateinsulator, the method comprising: forming a gate insulator overlying achannel region; forming a gate electrode including: a first metal layer,which is a barrier, overlying in the gate insulator; and, a second metallayer overlying the first metal layer; wherein the gate electrode has awork function selected from a group consisting of a high work functionand a low work function; wherein the gate electrode has a high workfunction exclusively responsive to the second metal layer being selectedfrom a group consisting of Ir, Re, Ni, Mn, Co, RuO2, Pd, Mo, and TaSiN;and, wherein the gate electrode has a low work function exclusivelyresponsive to the second metal layer being selected from the groupconsisting of Nb and NbO.
 13. In a MOSFET transistor with a reactivemetal gate electrode, a method for protecting the gate electrode from anunderlying gate insulator, the method comprising: forming a gateinsulator overlying a channel region; forming a gate electrodeincluding: a first metal layer of WN, which is a barrier, overlying thegate insulator, having a thickness of less than 5 nanometers (nm); and,a second metal layer overlying the first metal layer; wherein the gateelectrode has a work function exclusively responsive to a second metallayer selected from a group consisting of Pt, Pd, Nb, and NbO.